At 10-3, the San Francisco 49ers are fighting for the No. 2 seed in the NFC. With two losses in their last three outings, questions are starting to lurk. Are the Niners indeed a top-tier club with a powerhouse defense and limited-but-fundamentally sound offense? Or are they, like the ’08 Dolphins or 08 Titans, just another middle-tier team that happened to rack up a lot of wins thanks to the good fortunes of turnover differential? (The Niners are currently first in the league at +21).

San Fran’s recent two losses have been to quality 3-4 defenses (Baltimore and Arizona). The Monday night matchup against Pittsburgh could provide the “moment of truth” for Jim Harbaugh’s club.

1. Niners’ protection woesThe Cardinals defense, led by former Steelers assistant Ray Horton, came after Alex Smith & Co. with fervidity and dimension. Horton’s panoply of blitzes brought rushers from all four linebacking spots and, on a few occasions, the secondary. San Francisco’s offensive line, particularly inside with LG Mike Iupati, C Jonathan Goodwin and RG Adam Snyder, floundered in their identification and reaction speed. Two weeks before, those three linemen, along with backup guard Chilo Rachal, were physically manhandled by Haloti Ngata and the tough Ravens front three.

The Niners spend most of their time in base offensive personnel, which has them line up against base defensive personnel. The Steelers are less aggressive than the Cardinals when it comes to blitzing out of base personnel (most of Dick LeBeau’s blitzes come from nickel and dime packages). And, physically, the Steelers defensive front three is not as powerful as the Ravens’.

That said, the trenches mismatch will still be glaring and hard for the Niners to avoid (see items 2 and 3).

2. Niners run gameJim Harbaugh’s is a run-oriented offense in the purist form. On first and second downs, the 49ers align almost exclusively in 21 or 22 personnel (i.e. two backs and one or two tight ends). The Steelers, at times, even in their base defense with vociferous nose tackle Casey Hampton, have uncharacteristically struggled in run defense this season. But those struggles have come against zone-blocking teams like the Texans, Ravens or Bengals.

The 49ers are a power-blocking team. Their ground game is predicated on size and force, double-teams and interior pulls (Iupati is very mobile; Snyder is often ineffective off movement but can at least physically execute the plays). Power-blocking is not a good formula when facing the Steelers. Their defensive line cannot be consistently driven, and inside linebackers Lawrence Timmons and James Farrior play too fast for slow developing pull blocks to work.

3. Niners pass gameIf the Niners do try to stick with their power ground game, they’ll inevitably face a handful of third-and-long situations. That will compel Harbaugh to spread into three-receiver sets. That’s when LeBeau will take advantage of San Francisco’s interior pass protection issues.

One of the hallmark blitzes in LeBeau’s portfolio is the Fire-X, which is when both inside linebackers crisscross and attack the A-gaps. The Steelers execute Fire-X’s better than any team in football. James Farrior is brilliant in timing his blitzes and setting up pass-rushing lanes for teammates. Lawrence Timmons is more explosive than Acetone Peroxide when firing downhill.

Because rushing yards could be tough to come by, it’s very likely that the Niners will throw on early downs out of base personnel (they had success with this formula against the Giants a few weeks ago). To help Alex Smith thrive in these scenarios, Harbaugh has implemented several changes this season – such as using play-action and specific route designs that allow for one-read throws, eliminating sight adjustment routes to ensure that the receivers and quarterback are always on the same page and being very judicious in calling “shot plays” downfield.

But in most games, there are points when a quarterback and his receivers simply have to make things happen. Smith doesn’t have the dynamic tools to consistently do that against a D like Pittsburgh’s. His primary wide receivers don’t have the speed and quickness to regularly separate outside (especially against a star cornerback like Ike Taylor). And, most concerning, his offensive tackles, particularly lackluster second-year pro Anthony Davis, are not formidable enough in pass protection to stave off LaMarr Woodley or even Jason Worilds.

4. Niners defensive line vs. Steelers O-lineThe good news for Harbaugh is his defense is capable of posing nearly just as many problems for the Steelers offense. Obviously, Ben Roethlisberger’s health will have a significant impact on this game. You already know the advantages Big Ben gives the Steelers.

Almost as important is the health of center Maurkice Pouncey. Like Roethlisberger, he’s battling a Grade 1 high ankle sprain. Pouncey could not finish the game against Cleveland but says he’ll play Monday night. That’s huge. Without Pouncey, the Steelers would have to slide Doug Legursky from left guard to center, which poses a substantial drop-off in mobility and strength (even if Legursky has been somewhat of an overachiever the last year).

What’s more, Chris Kemoeatu would be forced back into the lineup at left guard. Kemoeatu has been a top ten player at his position the past few years. But for whatever reason, he’s fallen flat on his face this season – mainly in pass protection, where he’s shown poor lateral agility and a proclivity for holding.

Even at full strength, the Steelers offensive line is average and, thus, incapable of completely neutralizing the 49ers front line over four quarters. Left end Justin Smith is as good as they get. Nose tackle Isaac Sopoaga has blossomed into a plugger who’s mobile enough to make plays anywhere in the box.

Right end Ray McDonald is healthy again and flashing uncommon initial quickness. And on passing downs, Ahmad Brooks and Aldon Smith are lightning fast, supple edge-rushers with versatile short-area explosiveness. It’s highly doubtful the Steeler tackles can contain them one-on-one.

5. San Francisco’s defensive back sevenEven if Patrick Willis’ hamstring keeps him out a third-straight game, the Niners have enough speed and burst with NaVorro Bowman and strong safety Donte Whitner to answer Pittsburgh’s methodical rushing attack. The key will be whether San Francisco can hold up in pass defense. The Niners like to play zone in base D and man in nickel or dime.

Without Willis, San Francisco’s zones become somewhat vulnerable inside (we saw this on Early Doucet’s 60-yard touchdown last week). In man, Carlos Rogers, Tarell Brown and Chris Culliver are all capable of hanging with Antonio Brown, Emmanuel Sanders and Mike Wallace, but not if Roethlisberger is able to extend the play (Brown is simply too good at making late adjustments to his route, Sanders is similar and Wallace obviously has lethal speed if he can get downfield).